1. Field of the Invention
This invention is concerned with apparatus that permits utilization of a voltage penetration screen CRT as a rastered, multicolor device. More particularly, this invention is concerned with employing a voltage penetration CRT to obtain, at least, limited multicolor rastered images without the use of convergence or alignment electronics.
2. Description of the Prior Art
Voltage penetration screen cathode ray tubes, or CRTs, having a single electron gun, are well known in art. Such CRTs, since they have only one electron beam to scan, have no need for and, therefore, do not use either the alignment or convergence electronics that are necessary in conventional three gun color systems. Typically, their screen receives a multiple layer phosphor coating that enables a cathode ray tube with a single electron gun to display information in up to four colors. The actual color that is displayed will depend upon the amplitude of the beam accelerating voltage employed. The overall quality of the display has resolution comparable to that of a high quality monochrome tube employing conventional raster or scanning techniques. The data is screen displayed by what has been variously termed random scan, x-y writing or stroke writing. As these names imply, the data is written to the screen by precisely deflecting the CRT beam to predefined locations and not, as in conventional monitors or displays, by sweeping the beam across the screen.
One significant problem that this type of CRT has had is that of switching the rather high electron beam accelerating voltages quickly enough during vertical retrace to obtain a rapid change in color. That type of problem has been addressed by solutions of the type proposed in U.S. Pat. Nos. 4,092,566 and 4,203,055, both of which were issued to Chambers et al, and are concerned with rapid switching high voltage power supplies for a beam penetration color CRT. Thus, the problems related to the rapid switching of relatively high voltage levels to obtain quick color changes were soon solved and the penetration screen type of CRT became rather popular for certain uses.
This type of CRT has proven useful, for example, in applications ranging from air traffic control and cockpit displays to industrial process control and public utility power dispatching. It has been found that the colors used in these displays, the phosphor composites of which are selected to eliminate blue, closely match the spectral response of the human eye and allow extended viewing with greatly reduced visual fatigue.
However, there were some situations in which this type of CRT could not be effectively employed. In particular, a use limitation was brought about in the situation where the amount of information to be written to a screen was too great to be written before the screen had to be refreshed. In such a situation, some material placed on the screen began to or actually did fade before refresh could be accomplished. When the screen was subsequently refreshed, the tardy replacement or overwriting of this information made the screen images appear to flicker.
The use of high or, more accurately, longer persistent phosphors minimized the flicker problem since it now took longer for a screen image to fade. However, these phosphors were more expensive than the ordinary composites and suitable only for limited use. Furthermore, their use negated the economic advantages of employing the penetration screen type of CRT which allowed the designer to avoid convergence and alignment electronics and reap the savings attendant thereto. In addition, the emergence of low end or microprocessor based computing systems fueled a need for an economic and relatively high resolution multicolor CRT display that conventional shadow mask CRTs could not fulfill. While the penetration screen CRT seemed to promise an ability to satisfy that need, its inability to economically provide flicker-free displays of crowded screens had to be overcome.